Cancel an attachment (payments-api provider contract)

Called when an intent is cancelled or an attempt is displaced or expired. Withdraws a pending redirect: the attachment is closed here first, then what the wallet still holds for it is taken back — a one-time payment the payer has not completed is cancelled at the wallet, an unfinished activation's wallet link is deleted and its instrument ended, a renewal holds nothing. A wallet that will not or cannot let go does not undo the cancel: nothing charges a closed attachment, and the refusal is logged for reconciliation. Answers the attachment as it stands: cancelled, or the final state that got there first (a paid attachment stays succeeded; a late payment after a cancel is still reported as payment_intent.payment_created). Idempotent. An attachment of another account, or of the other mode, is 404.